1. Field of the Invention
This invention relates to liquid crystal displays and, more particularly, to a method and apparatus for improving the contrast and gray scale of an image during wide angle viewing of a liquid crystal display.
2. Prior Art
Liquid crystal displays (LCDs) include a thin film of a liquid crystal sandwiched between two transparent sheet electrodes. The sheet electrodes commonly used are glass which is coated with a thin film of a metal or metal oxide. The thickness of the liquid crystal film is in the range of about 6-25 microns and is controlled by a spacer which is chemically inert. The cell thus formed is then hermetically sealed. The cell is then placed between crossed polarizer sheets. Polarized light, passing through one of the crossed sheet polarizers, enters the liquid crystal layer and is rotated 90 degrees as it passes through the liquid crystal layer and passes through the second (crossed) polarizer sheet. If a mirror is placed behind the second sheet polarizer, the polarized light will be reflected back through the liquid crystal layer and the first polarizer sheet and provide a white background. When a voltage is applied across the liquid crystal layer, the long axis of those liquid crystals disposed between the electrodes align in the direction of the electric field. In this orientation, the liquid crystal molecules do not rotate the polarization state of light and the display appears dark against a white background.
A xe2x80x9csheet polarizerxe2x80x9d of the type referred to above is a polymeric film used for producing polarized light from unpolarized light. An example of a sheet polarizer that is commonly used for producing polarized light is a stretched sheet of a colorless polymeric film treated with an iodine solution. The most commonly used polarizer sheet includes a plurality of chains of polymer molecules such as, for example, polyvinyl alcohol (PVA), wherein each of the PVA polymer chains has a long axis which is parallel to the long axis of all other PVA chains, and containing conductive iodine atoms. In conventional liquid crystal displays (LCDs), the long (absorption) axes of the polymer chains are parallel to the plane containing the surface of the polarizer sheet. This is a result of the manufacturing process (e.g., unidirectional stretching), which leads to a uniaxial alignment of the microscopic iodine crystals in the polymer film (e.g., iodine-polyvinyl, iodine-polyvinylene). These microscopic iodine crystals are needle-like with the absorption (long) axis of the polymer chains parallel to the long axis of the needles. FIG. 1(a) shows a schematic drawing of the alignment of the microscopic iodine crystals in the polarizer.
Contrast and stability of gray scale levels are important attributes in determining the quality of a liquid crystal display. High quality (contrast, gray scale stability) information display can be obtained only within a narrow range of viewing angles centered about the normal incidence by using conventional LCDs. The narrow viewing angle has been a significant problem in advanced applications requiring high quality displays, such as avionics displays and wide-screen displays.
Yeh et al., in U.S. Pat. No. 5,196,953, the content of which patent is incorporated herein by reference thereto, discloses a means for improving the image of an LCD at large viewing angle. Yeh et al. suggest that the loss of contrast with increased viewing angle, in a normally white LCD, is, at least in part, because the homeotropic liquid crystal layer does not appear isotropic to off-normal light incident thereon. Yeh et al., in 953 , suggest the addition of an optical C-plate compensator to the LCD. A C-plate compensator is a negatively uniaxial birefringent plate with its extraordinary axis (i.e., its c-axis) perpendicular to the surface of the plate. Winker et al., in U.S. Pat. No. 5,986,733, employs a positive birefringent compensator layer, oriented with its optical axis substantially parallel to the average direction of the optic axis within a central, nominally homeotropic portion of the liquid crystal layer, in its driven state, disposed between the crossed sheet polarizer layers and the liquid crystal layer being tilted with respect to the parallel planes of the crossed polarizer sheets and the compensator layer. Notwithstanding the foregoing efforts to improve the wide-angle viewability of LCDs, there remains a need for low cost LCDs having a wide viewing angle.
It is an object of the invention to provide a polarizing film for use with a liquid crystal display that enables the wide-angle viewing of the image on the liquid crystal display without substantial loss of image contrast.
It is a further object of the invention to provide a method for making a liquid crystal display wherein the image on the display may be viewed over a large range of angles with minimum loss of image contrast.
The above objectives of the invention are met by employing a novel polarizer sheet in liquid crystal displays. The novel polarizer sheet is comprised of a plurality of parallel polymer chains; each chain having a long axis that is tilted with respect to a direction normal to the liquid crystal cell. In a twisted nematic LCD, the replacement of the standard analyzer polarizer sheet with a polarizer sheet wherein the polymer chains have a tilt angle of 30 degrees provides a significant improvement in the contrast ratios and gray scale stability for wide-angle viewing. Tilted polarizers can be easily implemented in projection displays by actually tilting the plane of a standard sheet polarizer relative to the plane of the LC cell. For flat panel LCD""s, tilted polarizers must be fabricated by physically (or chemically, or optically) tilting the c-axis (absorption axis) of the polarizer material relative to the film surface.